As compared with traditional wire-based networks, optical fiber communication networks are capable of transmitting significantly more information at significantly higher speeds. Optical fibers, therefore, are being increasingly employed in communication networks.
To expand total transmission throughput, optical fiber network providers are attempting to place ever more optical fibers in ever-smaller spaces. Packing fibers into tight spaces, however, can cause undesirable attenuation. Indeed, there is an inherent trade-off between increased fiber density and signal attenuation. Accordingly, a need exists for increasing the fiber density within loose buffer tubes (i.e., the buffer-tube filling coefficient) without causing undesirable attenuation.
Another issue that has been encountered is that reduced-size cables are often desirable for certain installations (e.g., where space is limited). In this regard, reduced-size cable designs are requiring ever-smaller buffer tubes. As buffer tubes become increasingly small, however, excess fiber length (EFL) becomes a significant problem. As will be known by those having ordinary skill in the art, EFL can occur as a result of buffer tube shrinkage during processing and thereafter as post-extrusion shrinkage (PES), which can lead to undesirable attenuation. In this regard, it is believed that smaller buffer tubes shrink more than larger buffer tubes under the same conditions.
U.S. Pat. No. 7,373,057 (Pizzorno) proposes to achieve highly reduced cable diameters by employing bend-resistant optical fibers having, at a wavelength of 1550 nanometers, microbending sensitivity of less than 4.0 (dB/km)/(gf/mm) when measured by an expandable drum apparatus at a temperature between about −30° C. and 60° C. In effect, U.S. Pat. No. 7,373,057, which is hereby incorporated by reference in its entirety, achieves higher filling coefficients and fiber densities by simply employing microbend-insensitive optical fibers. U.S. Pat. No. 7,373,057 does not address the problem of post-extrusion shrinkage (e.g., during mid-span deployment), and thus its high-density design is inappropriate for standard single-mode fibers (SSMF) in which mid-span accessibility is required (i.e., SSMF at elevated buffer-tube filling coefficients).
Buffer-tube designs having somewhat higher optical fiber densities have been achieved for the deployment of standard single-mode-fibers. For example, as many as 12 discrete, conventional optical fibers (e.g., SSMFs having a diameter of about 245-255 microns) have been deployed in loose buffer tubes with an outer diameter larger than 2.5 millimeters and an inner diameter larger than 1.6 millimeters. For SSMFs, however, as the buffer-tube filling coefficient approaches 0.3, attenuation becomes problematic, particularly at extreme temperatures (e.g., −40° C. or 70° C.). This is especially so with respect to mid-span storage performance, such as deployments in which SSMFs are positioned in pedestals, cabinets, or other optical-fiber enclosures. By way of example, loose-tube cables must be accessible multiple times along its installed length at various positions, typically at such optical-fiber enclosures.
By way of illustration, after installation in a microduct, an optical-fiber cable typically experiences temperature cycles. These temperature cycles can lead to signal attenuation. Indeed, significant changes in temperature can lead to post-extrusion shrinkage and increases in excess fiber length (EFL), which may contribute to signal attenuation. Thus, a loose buffer tube that is less susceptible to post-extrusion shrinkage is more suitable for mid-span storage. It is generally thought that cables containing buffer tubes having a lower buffer-tube filling coefficient are less susceptible to attenuation when subjected to temperature cycles and thus are more suitable for mid-span storage.
Reducing the wall thickness of a buffer tube while maintaining its outer diameter necessarily increases its inner diameter and thus the cross-sectional area available for deploying optical fibers. For many optical fiber applications, reducing buffer-tube wall thickness is unsatisfactory because such buffer tubes provide insufficient crush resistance (i.e., hoop strength). For many rigorous applications, buffer tubes must be capable of handling loads during installation and use in a way that conforms to customer expectations.
Despite efforts to achieve reduced-diameter buffer tubes, a need exists for buffer-tube designs that provide satisfactory mid-span storage and crush resistance for both standard single-mode fibers and bend-insensitive optical fibers.